1. Field of the Invention
The present invention relates to an oil pump for an engine lubricant and, more particularly, to a trochoidal oil pump for effecting pumping actions in accordance with changes in the capacity of a sealed space between inner and outer rotors.
2. Description of the Prior Art
One example of the oil pump of the above-specified kind is disclosed in Japanese Patent Laid-Open No. 58-35212. In this trochoidal oil pump, a housing is fixed to form an internal space together with the side of an engine block. As shown in FIG. 7, an annular outer rotor 50 is fitted rotatably in the internal space of said housing. In this outer rotor 50, there is fitted an inner rotor 51 which has a four-lobed trochoidal curve and which is formed with external teeth 51a meshing with the internal teeth 50a of said outer rotor 50. The inner rotor 51 is driven by a rotating shaft which is coupled thereto and borne by the aforementioned housing. Deeper than the bottom of the internal space of the housing, there are formed a suction chamber 52 and a discharge chamber 53, both of which communicate with the internal space of the housing. Oil is discharged in accordance with changes in the capacity of a sealed space (as hatched in the drawing) 54 which is defined by the internal teeth 50a and the external teeth 51a.
On the other hand, the oil pump disclosed in Japanese Patent Laid-Open No. 58-70014 is of the type, in which it is connected directly to a crankshaft having a multi-toothed trochoidal rotor. In the internal space of a housing, more specifically, there is rotatably fitted an annular outer rotor 60 which is shown in FIG. 8. In this outer rotor 60, there is fitted in inner rotor 61 which has its external teeth 61a meshing with the internal teeth 60a of the outer rotor 60. This inner rotor 61 is driven directly by the crankshaft. Deeper than the bottom of the internal space of the housing, are formed a suction chamber 62 and a discharge chamber 63, both of which communicate with the aforementioned internal space. Thus, oil is discharged to the discharge chamber 63 in accordance with changes in the volume of a sealed space (as hatched in the drawing), which is defined by the internal teeth 60a and the external teeth 61a.
The aforementioned oil pumps of the prior art will be described in connection with the example of FIG. 8 by way of example. The discharge chamber 63 is formed to extend in the direction of rotation (i.e., a counter-clockwise direction) from a contact position, in which the internal and external teeth come into first contact, with respect to the dedendum 65 (or 55 in the other example of the prior art shown in FIG. 7) of the inner rotor 61 at the maximum of the sealed space 64. In other words, the opening of the discharge chamber 63 into the internal space of the housing is formed to extend from the above-specified contact position 66 in the direction of rotation. In this oil pump of the prior art, however, as soon as the inner rotor 61 rotates, communication between the discharge chamber 63 and the sealed space 64 is instantly initiated to have its area enlarged abruptly, and the discharge chamber 63 is formed axially within the housing. As a result, under the influence of the discharge pressure of the chamber 63, the oil flows back from the discharge chamber 63 into the sealed space 64 so that a higher peak pressure than the discharge pressure is built up with fluctuations in the sealed space 64. These pressure fluctuations propagate with the resultant fluctuations in the rotations of the two rotors, thus raising a problem in that noises or wear of the teeth are caused by the resonance or the like of the hydraulic circuit inclusive.